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Abstract:

A thin-film deposition system has a vacuum chamber and a plasma generator.
The plasma generator includes a case, a cathode disposed in the case, an
anode assembly disposed at an end of the case, a discharge power supply
for applying a discharge voltage between the cathode and the anode
assembly, and a gas supply means for supplying a discharge gas into the
case. Electrons within a first plasma produced in the case are extracted
into the vacuum chamber according to the discharge voltage. An evaporated
material in a gaseous state inside the vacuum chamber is irradiated with
electrons emitted from the plasma generator to produce a second plasma.
The potential at the anode assembly is controlled by anode
potential-controlling means such that the electrons within the second
plasma are directed at the plasma generator and the ions within the
second plasma are directed at the substrate.

Claims:

1. A thin-film deposition system comprising:a vacuum chamber; anda plasma
generator including a discharge chamber, a cathode disposed in the
discharge chamber, an anode assembly disposed at an end of the discharge
chamber, a first power supply for applying a discharge voltage between
the cathode and the anode assembly, and means for supplying a discharge
gas into the discharge chamber,wherein electrons within a first plasma
produced in the discharge chamber are extracted into the vacuum chamber
through the aperture of the anode assembly according to the discharge
voltage, the vacuum chamber being located outside the discharge
chamber,wherein an evaporable material that is at least in a gaseous
state within the vacuum chamber is irradiated with electrons emitted from
the plasma generator to produce a second plasma, thus forming a film on a
substrate, andwherein there is further provided anode
potential-controlling means for controlling the potential at the anode
assembly such that the electrons within the second plasma are directed
toward the plasma generator, whereby ions within the second plasma are
directed at the substrate.

2. A thin-film deposition system as set forth in claim 1, wherein said
anode potential-controlling means is capable of controlling energy of the
ions bombarded against the substrate.

3. A thin-film deposition system as set forth in claim 1, wherein said
anode potential-controlling means maintains the potential at the anode
assembly at a positive value with respect to ground potential at all
times.

4. A thin-film deposition system as set forth in claim 1, wherein(A) said
anode potential-controlling means is formed by connecting an
impedance-adjusting circuit and an auxiliary power supply in series
between the anode assembly and a grounded point,(B) a positive bias is
applied to the anode assembly at all times,(C) when an electron current
flows into the grounded point, the impedance-adjusting circuit produces a
given impedance to make an adjustment for limiting the electron current,
and(D) when an ion current flows into the grounded point, the impedance
is made zero and the ion current is permitted to flow.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a thin-film deposition system
having a vacuum chamber containing a plasma generator.

[0003]2. Description of Related Art

[0004]A plasma generator converts gaseous molecules and evaporated
particles into a high-density plasma in a thin-film deposition system,
such as an ion plating system, for forming a thin film on a substrate and
assisting in impinging the ions contained within the plasma onto the
substrate.

[0005]FIG. 3 schematically shows one example of a thin-film deposition
system incorporating a plasma generator. A crucible 3 holding an
evaporable material 2 therein is mounted at the bottom of a vacuum
chamber 1. An electron gun 4 for emitting an electron beam toward the
evaporable material is also mounted at the bottom of the chamber 1.

[0006]A rotatable substrate dome 5 on which plural substrates are set is
mounted near the top of the vacuum chamber 1. The dome is equipped with a
heater 6.

[0007]A plasma generator 7 emits an electron beam to the space between the
substrate dome 5 and the crucible 3.

[0008]The plasma generator 7 includes a cathode 8 made of thermionic
tungsten filament or other material. The cathode 8 is connected with a
heater power supply 9. An electric discharge chamber is formed inside a
cylindrical case 10. The pressure inside the discharge chamber is made
higher than the pressure inside the vacuum chamber 1 by argon gas
introduced from a gas inlet port 11.

[0009]A first anode 12 is water-cooled and connected with a discharge
power supply 13 via a resistor R1 having resistance R1. A second anode 14
is mounted to cover the surface of the first anode. A part of the first
anode 12 is connected to a part of the second anode 14 to increase the
thermal resistivity between them.

[0010]A shield body 15 is held to the case 10. An orifice permitting
passage of the electron beam is formed at the front end of the shield
body 15. A coil 16 consists of an electromagnet for producing a magnetic
field parallel to the direction in which electrons are extracted. A
plasma 17 created in the case 10 is focused toward the center axis of the
case by the coil 16.

[0011]The cathode 8, first anode 12, second anode 14, case 10, resistor
R1, heater power supply 9, and discharge power supply 13 together form a
discharge circuit, which in turn is connected with the vacuum chamber 1
via a resistor R2 having resistance R2.

[0012]In the plasma generator 7, a given amount of argon gas is first
introduced into the case 10 from the gas inlet port 11 to increase the
pressure inside the case. The cathode 8 is heated to a temperature at
which thermionic emission is possible by the heater power supply 9. Then,
the coil 16 is energized with a given electrical current to induce plasma
ignition and produce a magnetic field necessary to obtain a stable
plasma.

[0013]Under this condition, if a given voltage, for example, of 100 V is
applied between the cathode 8 and the anode assembly (12, 14) from the
discharge power supply 13, an electric field 18 is produced over the
orifice formed in the shield body 15. Thermoelectrons emitted from the
cathode 8 are started to be accelerated toward the anode assembly (12,
14) by the electric field. The acceleration of the thermoelectrons causes
repeated collisions of the thermoelectrons with the introduced argon gas,
producing the plasma 17 inside the case 10.

[0014]The electrons produced inside the plasma 17 in this way are drawn
into the vacuum chamber 1 by the electric field 18 while focused toward
the center axis of the case by the magnetic field produced by the coil
16.

[0015]On the other hand, inside the vacuum chamber 1, an electron beam 19
from the electron gun 4 is directed at the evaporable material 2. The
material is heated and evaporated. A process gas (e.g., oxygen gas) is
introduced into the vacuum chamber 1 from a process gas inlet port 20.

[0016]The electrons extracted into the vacuum chamber 1 are made to
collide against the process gas and particles of the evaporated material
inside the vacuum chamber. The gas and particles are excited and ionized.
Consequently, a plasma 22 is produced inside the vacuum chamber. The
evaporated particles ionized within the plasma are drawn to the substrate
set on the substrate dome 5 and adhered to the substrate. A film of the
particles of the evaporated material is formed on the substrate.

[0017]The electrons extracted into the vacuum chamber 1 and the electrons
within the plasma 22 flow into the wall of the vacuum chamber 1 and into
the anodes 12, 14, maintaining a stable electric discharge.

[0018]Where an optical thin film is formed by the thin-film deposition
system designed as described above, particles of evaporated,
non-conductive dielectric materials that form the optical thin film
adhere to the inner wall of the vacuum chamber 1, increasing the
impedance. Therefore, most of the electrons extracted into the vacuum
chamber 1 and the electrons within the plasma 22 are forced toward the
anodes 12 and 14 by establishing the relationship R1<R2, where R1 and
R2 are the resistances of the resistors R1 and R2 of the plasma generator
7.

[0019]The process by which a film is formed on the substrate is next
described in somewhat further detail. The plasma 22 created in the vacuum
chamber 1 by the plasma generator 7 gives energy to the particles
evaporated from the evaporable material 2 and the oxygen gas from the
process gas inlet port 20. Some of them are excited and ionized. As a
result, the high-density plasma 22 is created inside the vacuum chamber
1. Furthermore, electrons are accumulated on the surface of the substrate
dome 5 exposed to the plasma 22. A negative voltage is applied to the
surface of the substrate dome.

[0020]Meanwhile, the plasma 22 has a zero or positive potential and so
there is a difference in potential between the plasma 22 and the
substrate dome 5 near the surface of the dome 5. The ions within the
plasma 22 near the substrate dome are accelerated toward the substrate,
thus bombarding it.

[0021]The bombardment is combined with the excitation and ionization of
the evaporated particles and process gas to permit the quality of the
film formed on the substrate to be improved. That is, the packing density
of the film is enhanced, and the adhesion is improved.

[0022]Where an optical thin film is formed by a thin-film deposition
system, the film is strongly required to have optical characteristics
which do not change with environmental variations. For this purpose, it
is important to enhance the packing density of the film. In the
above-described thin-film deposition system, bombardment of ions present
close to the substrate dome 5 against the substrate greatly contributes
to the packing density.

[0023]Accordingly, in the aforementioned thin-film deposition system, the
difference between the negative potential at the substrate dome 5 and the
positive potential possessed by the plasma 22 is increased by increasing
the density of the plasma 22. This increases the energy with which the
ions present close to the dome 5 are accelerated toward the substrate. As
a result, the packing density of the film can be enhanced. An optical
thin film having improved environmental resistance can be formed.

[0024]However, if the density of the plasma 22 within the vacuum chamber 1
is enhanced, the temperature of the substrate dome 5 exposed to the
plasma is elevated greatly with the elapse of time. There is the danger
that the maximum processing temperature of the substrate will be
exceeded.

[0025]If coating is done at a temperature lower than the maximum
processing temperature of the substrate by lowering the density of the
plasma 22, ions accelerated toward the substrate have lower energies, and
the packing density of the film is not enhanced. There is the problem
that the quality of the film is deteriorated.

[0026]It is desirable to be capable of modifying the energy of the ions
bombarded against the substrate at will according to the kind of the
evaporable material. In the present system, however, it is impossible to
control the density of the plasma 22 within the vacuum chamber and the
energy of the ions accelerated toward the substrate independently. Hence,
it is not possible to finely establish the conditions under which thin
films are formed.

SUMMARY OF THE INVENTION

[0027]It is an object of the present invention to provide a novel
thin-film deposition system capable of solving the foregoing problems.

[0028]A thin-film deposition system according to one embodiment of the
present invention includes a vacuum chamber and a plasma generator. The
plasma generator has: a discharge chamber; a cathode disposed in the
discharge chamber; an anode assembly disposed at an end of the discharge
chamber; a cylindrical shield body mounted in the discharge chamber and
surrounding at least the anode assembly out of the anode assembly and the
cathode; a first power supply for applying a discharge voltage between
the cathode and the anode assembly; and a port for supplying a discharge
gas into the discharge chamber. Electrons within a first plasma produced
in the discharge chamber are extracted into the vacuum chamber through a
part of the shield body according to the discharge voltage, the vacuum
chamber being located outside the discharge chamber. An evaporable
material that is at least in a gaseous state within the vacuum chamber is
irradiated with electrons emitted from the plasma generator to produce a
second plasma, thus forming a film on the substrate. The potential at the
anode assembly is so controlled by an anode potential controller that the
electrons within the second plasma are directed toward the plasma
generator, whereby ions within the second plasma are directed at the
substrate.

[0029]According to the present invention, increases in temperature of the
substrate are suppressed by using a low-density plasma. The substrate can
be irradiated with an arbitrary energy beam created by ions within the
low-density plasma. The packing density of a thin film formed under
low-temperature process environments can be enhanced. A high-quality thin
film can be built.

[0030]Furthermore, generation of the plasma within the vacuum chamber and
the energy with which the ions within the plasma are bombarded against
the substrate can be controlled independently. Therefore, an ion beam
having an optimum energy can be bombarded against the substrate according
to the kind of the evaporable material. As a result, conditions under
which each evaporable material is vapor-deposited can be set finely.

[0031]These and other objects and advantages of the present invention will
become more apparent as the following description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic representation of a thin-film deposition
system according to one embodiment of the present invention, the system
incorporating a plasma generator;

[0033]FIG. 2 is a graph illustrating potentials at various locations
within the system shown in FIG. 1; and

[0034]FIG. 3 is a schematic representation of a prior art thin-film
deposition system incorporating a plasma generator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035]The preferred embodiment of the present invention is hereinafter
described in detail with reference to the drawings.

[0036]FIG. 1 is a schematic representation of a thin-film deposition
system according to one embodiment of the present invention. In both
FIGS. 1 and 3, like components are indicated by like reference numerals.

[0037]The system shown in FIG. 1 is similar to the system already
described in connection with FIG. 3 except that the discharge circuit
composed of the cathode 8, first anode 12, second anode 14, case 10,
resistor R1, heater power supply 9, and discharge power supply 13 is
connected with the vacuum chamber 1 via an impedance-adjusting circuit 23
and an auxiliary power supply 24. The chamber 1 is at ground potential.
The system constructed in this way operates as follows.

[0038]First, a given amount of argon gas is introduced into the case 10
from the gas inlet port 11 to increase the pressure inside the case. The
cathode 8 is heated to a temperature where thermionic emission is
possible by the heater power supply 9.

[0039]Then, the coil 16 is energized with a given electrical current to
induce plasma ignition and produce a magnetic field necessary to obtain a
stable plasma.

[0040]Under this condition, if a given voltage, for example, of 100 V is
applied between the cathode 8 and the anode assembly (12, 14) from the
discharge power supply 13, an electric field 18 is produced over an
orifice formed in the shield body 15. Thermoelectrons emitted from the
cathode 8 are started to be accelerated toward the anodes 12 and 14 by
the electric field. The acceleration of the thermoelectrons causes
repeated collisions of the thermoelectrons with the introduced argon gas,
producing the plasma 17 inside the case 10.

[0041]The electrons produced inside the plasma 17 in this way are drawn
into the vacuum chamber 1 by the electric field 18 while focused toward
the center axis of the case 10 by the magnetic field produced by the coil
16.

[0042]Inside the vacuum chamber 1, the electron beam. 19 from the electron
gun 4 is directed at the evaporable material 2. The material is heated
and evaporated. A process gas (e.g., oxygen gas) is introduced into the
vacuum chamber from a process gas inlet port 20.

[0043]The electrons extracted into the vacuum chamber 1 are made to
collide against the process gas and particles of the evaporated material
inside the vacuum chamber. The gas and particles are excited and ionized.
Consequently, a plasma 22 is produced inside the vacuum chamber. The
evaporated particles ionized within the plasma are drawn to the substrate
set on the substrate dome 5 and adhered to the substrate. A film of the
particles of the evaporated material is formed on the substrate.

[0044]The electrons extracted into the vacuum chamber 1 and the electrons
within the plasma 22 flow into the wall of the vacuum chamber 1 and into
the anodes 12, 14, maintaining a stable electric discharge. The features
of the present invention are described below.

[0045]Under environments of a low-temperature process (e.g., below
150° C.), the thin-film deposition system according to the present
invention is utilized in the manner described below.

[0046]First, the output of the plasma 22 is set low. In this embodiment,
the discharge current is set to about 10 A by controlling the discharge
power supply 13. At this current, increases in the substrate temperature
are not affected. The discharge voltage is merely required to produce the
discharge current. Normally, the discharge current is set to an arbitrary
value within the range from about 70 V to 140 V depending on the pressure
condition, because the discharge current is used under constant-current
control.

[0047]Under this condition, the plasma 22 created inside the vacuum
chamber 1 has a low density causing less increase in the temperature of
the substrate. In this state, ions present near the substrate dome 5 are
accelerated toward the substrate with low energy, and it is impossible to
enhance the packing density of the formed film as described previously.

[0048]Accordingly, the impedance-adjusting circuit 23 introduced by the
present invention limits the electron current I flowing from the grounded
portion (vacuum chamber 1) into the discharge power supply 13. This makes
it possible to produce any arbitrary resistance R (e.g., 20 to
600Ω) for enhancing the potential at the anode assembly (12, 14).
As a result, a self-biasing voltage (I×R) produced by the
impedance-adjusting circuit 23 makes the potential at the anode assembly
have a positive potential with respect to the potential at the grounded
portion (vacuum chamber 1) c, as indicated by b in FIG. 2.

[0049]FIG. 2 is a graph showing the potentials at various locations within
the system shown in FIG. 1. In the graph, the vertical axis indicates
potential, while the horizontal axis indicates position (i.e., distance).
The potential at the cathode 8 is indicated by a. Although the bias
voltage (I×R1) produced by the resistor R1 affects the potential at
the anode assembly (12, 14), the bias voltage generated by the resistor
R1 is prevented from becoming high by setting the value of R1 to less
than 10Ω. In FIG. 2, discharge voltage A is produced between the
anode assembly (12, 14) of the plasma generator 7 and the cathode 8.

[0050]The auxiliary power supply 24 produces a voltage that further
enhances the anode potential as indicated by e in FIG. 2 (at that time,
the potential at the cathode 8 is indicated by d). As a result, the
potential difference (auxiliary voltage) between the anode potential
indicated by e and the potential at the grounded portion (indicated by c)
increases as shown in FIG. 2. Electrons within the plasma 22 are
accelerated by the anodes 12 and 14 and, at the same time, the ions
within the plasma 22 are accelerated in the direction reverse to the
motion of the electrons. Thus, the ions move toward the substrate. When
the current flowing into the grounded vacuum chamber 1 is switched from
current based on electrons (electron current) to current based on ions
(ion current), the generated resistance R is controlled to zero by the
impedance-adjusting circuit 23. Under this condition, no limitations are
imposed on the ion current flowing through the impedance-adjusting
circuit 23.

[0051]In this way, the plasma 22 having an arbitrary density is created by
controlling the discharge power supply 13. Apart from this, the auxiliary
power supply 24 is controlled to accelerate the ions within the plasma 22
at will. An ion energy corresponding to the voltage generated by the
auxiliary power supply can be given to the substrate. Accordingly, a
low-density plasma can be created. The substrate can be bombarded with a
strong energy beam produced from the ions within the low-density plasma.
Consequently, in formation of a thin film, the packing density of the
film can be enhanced.

[0052]The discharge power supply 13 that is a power supply for creating
the plasma 22 in the vacuum chamber 1 cooperates with the auxiliary power
supply 24 for accelerating electrons within the plasma 22 toward the
anode assembly (12, 14) to enable independent control over generation of
the plasma 22 and the energy of ions contained within the plasma and
bombarding the substrate. In consequence, an ion energy beam that is
optimal for the kind of the evaporable material can be emitted. Hence,
evaporation conditions can be finely set for various evaporable
materials.

[0053]Having thus described our invention with the detail and
particularity required by the Patent Laws, what is desired protected by
Letters Patent is set forth in the following claims.